John james hood



' but the UNITED STATES PATENT OFFICE.

JOHN JAMES HOOD, OF LONDON, ENGLAND.

111i; EXTRACTING M ETALS.

SPECIFICATION forming part of Letters Patent No. 547,790, dated October15, 1895. Application filed January 9, 1895. Serial No. 534,369. (N0SPEQimGHSJ To aZZ whom it may concern.-

Be it known thatI, JOHN JAMES chemist, a subject of the Queen ofBritain, residing Hoon, Great at l Fenchurch Avenue, in

the city of London, England, have invented certain new and usefulImprovements in Extracting Metals and New Solvent Materials Therefor, ofwhich the following is a specification.

The improvements described herein relate to metallurgical processes forthe extraction of valuable metals from their ores, and have for theirobject not primarily the precipitation of a more valuable metal from itssolution at the expense of a less valuable metal,

solution ordissolving of a more valuable metal by the precipitation of aless valuable one. Such improvements have regard more particularly tothe precious metals gold and silver, which, as is well known, occur frequently in their ores in a metallic state. I employ for the extractionof such metals a solution of a salt or compound of a baser metal, whichon being brought into the presence of the precious metal (whether in ametallic state or otherwise) causes it to pass into solution by reasonof its displacing the baser metal in the solution. The extraction of theprecious metals is efiected by washing the ground ores with such asolution. Taking gold as an example, I will now describe several suchsolutions of metallic salts as will dissolve gold while the baser metalis precipitated and will set forth how the solutions are prepared andworked and the method of recovering the gold from the solutions. Ifmetallic gold or natural auriferous ores be treated with certainsolutions of salts of mercury or lead or other base metals, the gold isdissolved with great ease, the base metal being precipitated. The metalI prefer to employ is mercury; but lead does Well, and several othermetals-such as copper, arsenic, and antimony-will do, though lessconveniently. This preference is because the base metals like lead tendto form a hard covering upon the particles of gold and so to render thefurther solution of the precious metal slow, Whereas in the case ofmercury as it is precipitated on the particles mains fluid and hardly,if at solvent action of the solution of gold it reall, retards theemployed. To

chloride dissolves 1.45

prepare such an active solution in one WayI take the cyanide, sulphate,chloride, or oxide of mercury or other compound of mercury soluble in analkaline cyanide and dissolve it in a solution of asuitable alkalinecyanide, preferably soda or potash cyanide or mixtures of these. Alkali,either as caustic or carbonate, (by preference caustic soda,) is inpractice also added. If the material is being prepared for export, anyinsoluble matters are allowed to settle out and the clear solution isevaporated to dryness. This can readily be done, for the presence of themetallic salt prevents loss of cyanide during the evaporation. Amixturethat answers well consists of two parts, by weight, of cyanide ofpotassium, (or its equivalent of cyanide of sodium,) one part ofmercuric chloride or its equivalent of sulphate or other mercury salt,and from one-half to two parts of caustic soda. If a small quantity ofthis solution be placed upon gold, mercury is quickly deposited and goldgoes into solution.

Estimating the amount of gold dissolved for a given amount of mercuricchloride, the reaction appears to be in its first stage represented by2Au+3HgOl :3Hg-{-Ao Ol ,or, roughly, two parts, by weight, of mercuricchloride employed on a quantity of auriferous quartz with an alkalinecyanide solution will dissolve one part of gold.

If pure gold be treated with the above solution and kept warm, asecondaryreaction probably takes place, resulting in the amount of golddissolved being greater as compared with the mercuric chloride than theabove proportion. This may possibly be explained by the solvent actionof the auric salt acting on the metallic gold to form an aurous salt inthe presence of alkaline cyanide, just as a ferric salt will dissolvemetallic iron to form a ferrous salt, and substantially when a mercuricsalt (e. g., the chloride) is employed, the complete reaction appears tobe HgOl +2Au Hg-l-QAuGl; in other words, the mercuric parts of gold.Doubtless, also, secondary reactions take place between the aurouschloride and the potassic cyanide to form the double cyanide of gold andpotassium.

To prepare another base metal for dissolving gold-for instance, lead-Itake one of its ILO compounds, such as ordinary litharge, boil it withcaustic alkali to dissolve as much as possible, and mix this alkalinelead solution with the alkaline cyanide and employ the mixed solution,as in the case of mercury. Such a solution poured upon metallic goldsoon gives a black deposit of metalliclead, and gold goes into solution.These gold solvents may be converted into solid masses and packed forexport, or they may be prepared as required at the mines from theconstituents.

Although I have mentioned certain salts of baser metals in thepreparation of my solution, I do not restrict myself to these salts. Imention them because they are merely convenient and cheap salts to usein the preparation of my solution, which when prepared consists,essentially, of the cyanide of an alkali with some salt of a baser metalin solution; and in all these cases the precious metal when beingdissolved is acted upon by a solution which contains a baser metal orsome salt of a baser metal in solution in the presence of an alkalinecyanide, and all the solutions act upon the precious metals in the samewaythat is to say, by the precious metal displacing the baser metal inthat solution. Instead of preparing this metallic compound in the mannerabove indicated, in which caustic alkali is employed, I may omit thecaustic at this stage and proceed by reacting upon a soluble cyanidewith a compound of mercury soluble therein and, if desired,crystallizing it. The material thus obtained is employed in conjunctionwith caustic alkali, or, what yields better results, with a solution ofalkaline cyanide.

If the process be performed at the mines without the assistance of thecompound previously prepared, the solutions may be formed in the vatitself and in the presence of the ore. Thus, if to an ore lying in asolution of alkaline cyanide a mercury or lead salt soluble in suchsolution be added the gold will pass into solution, displacing the basermetal, or the material may be made directly at the mines by dissolving amercury or lead compound in alkaline cyanide just as required, employingabout two ounces or more of the mercury salt for each ounce of gold.

To employ any of the above-described solvent solutions, the ore isground in the usual fashion and placed inside tanks of wood, or if thetanks be of metal the inside and all the metallic connections are coatedwith paint or varnish or otherwise protected, so as to prevent themercury salt acting upon them, and the mass of ore is washed to free itfrom acid bodies, which would precipitate the mercury, or the washingmay be done simultaneously with the grinding. The alkaline solventsolution is poured over the ore and allowed to run through. It is againpoured over the ore and allowed to rest for several hours, and it isessential that the solution should still be alkaline as it runs from theore. The ore is next washed and the solutions and washings are treatedineither of the following methods: The auriferous solution is acidifiedwith sulphuric or other acid and run over iron filings or finely-dividediron, which precipitates the gold, or without acidifyingl employ thehighly active galvanic couple devised by the chemists Gladstone andTribe, known as the copper-zinc couple, which may be prepared, accordingto the methods described by them. either as a wet or dry couple. Thelatter I prefer, asit is more suitable for export and is made byagitating and gently heating a mixture of finely-divided copper withzinc. This highly reducing agent plays a double office when brought intocontact with the auriferous solution, as it not only reduces the gold tothe metallic state, but also any excess of mercury that maybe in thesolution. The zinc-copper couple acts well even in an acid solution.

In the practical working of the process two rules are to be observed:first, the solution is to be alkaline until the gold has displaced thebaser metal and the solution has been run off the ore, and, secondly,the alkalinity during the precipitation by the zinc-copper couple mustnot be so great as to prevent the precipitations taking place freely.The alkalinity may be regulated in the ordinary way by the addition of asufficient quantity of an alkali, such as caustic soda, or by partiallyneutralizing by acid.

The above process is applicable to ores where the gold is in a metallicstate as well as to those in which it is in chemical composition withother substances. Here I would remark that by the terms base metal andbaser metal I mean to denote mercury, lead, and such other metals as aredisplaced by metallic gold from their solutions in alkaline cyanide. Ifand so far as the gold is in the form of a sulphide in the ore, the useof the metallic salt is not required, as the sulphide of gold will passinto solution in an alkaline cyanide without any other metallic saltbeing present. I do not, therefore, claim the above method as applied tothe purpose of the extraction of gold .when in the form of a sulphide ofgold, which may be extracted or got rid of in any way desired, but onlyso far as the gold is in the shape of metallic gold or compound otherthan the sulphide; but, subject to the above limitations, I would haveit understood that What I claim is- 1. The solvent herein described, forthe extraction of precious metals from their ores, comprising a solutionof an alkaline cyanide and a salt or compound of a baser metal in theproportion of at least one part of the cyanide to two of the basermetal.

2. The method, substantially as hereinbefore set forth, for theextraction of precious metals from their ores, by the action of asolution containingalkali metal cyanide and a salt or compound of abaser metal in the proportion of one part at least of the former to twoparts of the latter, such treatment causing the precious metal todisplace the baser metal and thereby pass into solution.

3. The method, substantially as hereinbefore set forth, for theextraction of precious metals from their ores, which consistsin treatingthe ore with a solution containing both a cyanide of potassium or sodiumand a salt or compound of a baser metal in the proportion of one part atleast of the former to two parts of the latter; the metallic base of thesolution being displaced by the precious metal, the former beingprecipitated.

4. The method, substantially as hereinbesolution.

JOHN JAMES HOOD. Witnesses:

WILFRED CARPMAEL, THOMAS LAKE.

